1. Field of the Invention
The present invention relates to a storage system such as a RAID disk array apparatus and, in particular, to the improvement in the data caching technology for increasing the speed of responses to data input/output requests from a host device.
2. Description of the Related Art
A storage system such as a RAID disk array apparatus includes, for example, multiple channel adapters, many storage devices, multiple storage adapters, a cache memory, and a connection network. The multiple channel adapters communicate commands and data to a host device. The storage devices, such as a hard disk drive, store data. The multiple storage adapters communicate commands and data to the storage devices. The cache memory temporarily stores data exchanged between the multiple channel adapters and storage adapters. The connection network connects the multiple channel adapters and storage adapters to the cache memory. The cache memory is connected to the channel adapters and storage adapters such that any of the channel adapters and storage adapters can access the cache memory. Even when the data read/write speed of the storage devices is low, the speed of the responses to data read/write requests from the host device can be more increased by using the cache memory.
Various kinds of architecture can be adopted for the connection network for connecting the multiple channel adapters to the cache memory in the storage system. For example, JP-A-2000-339101 (FIGS. 1 and 2 and paragraphs 0005 to 0006 and the like) discloses the one, which can be called hierarchical starnet architecture. In this case, multiple channel adapters and a cache memory are connected to a selector, and the selector switches multiple communication paths between the cache memory and the multiple channel adapters. Also, a crossbar switch architecture for connecting the cache memory and the multiple channel adapters through a crossbar switch and another crossbar switch architecture for connecting them through a common bus are known. Furthermore, a connection network is known in which each of multiple channel adapters is directly connected to a cache memory through a one-to-one dedicated connection line.
In the storage system, upon arrival of a data read/write request from the host device, the data to be read/written is read to the channel adapter through the connection network from the cache memory and is transferred to the host device therefrom or is written in the cache memory from the host device through the opposite path. Therefore, the channel adapter and the connection network are controlled from the arrival of the request from the host device. Therefore, the response of the storage system to the host device delays by the time required for transferring the data from the cache memory to a buffer within the channel adapter through the connection network (or for transferring the data through the opposite path).